1. Field of the Invention
The present invention relates to an apparatus for controlling an elevator of a type which uses a linear induction motor to elevate/lower a car, and, more particularly, to an apparatus for controlling an elevator capable of providing enhanced comfort for passengers by compensating for propulsion changes and propulsion ripples.
2. Description of the Related Art
Hitherto, an apparatus for controlling an elevator has been known which has a linear induction motor composed of a primary coil (an armature), provided for a counterweight or a car, and a secondary conductor (conductor plate) provided in a shaftway to face the primary coil, the linear induction motor being able to generate propulsion with which the car is elevated/lowered.
In an apparatus of the aforesaid type, AC power is usually supplied to the linear induction motor from a variable voltage and variable frequency inverter thereof (hereinafter simply called an "inverter"). The inverter has a control circuit which feeds back the moving speed so as to subject it to a comparison with the commanded speed and controls the electric power to be supplied from the inverter to the linear induction motor in accordance with the obtained speed deviation.
FIG. 5 is a side elevational view which schematically illustrates a conventional apparatus for controlling an elevator disclosed in, for example, Japanese Patent Laid-Open No. 1-271381. FIG. 6 is a horizontal cross sectional view which illustrates a counterweight and a peripheral portion of a linear induction motor shown in FIG. 5.
Referring to FIGS. 5 and 6, reference numeral 1 represents a car, 2 represents a counterweight for balancing the weight of the car 1, and 3 represents a rope for suspending the car 1 and the counterweight 2 at extrimities thereof. The aforesaid elements 1, 2 and 3 constitute an elevation member disposed in the shaftway and moved together with the car 1.
Reference numeral 4 represents a sheave disposed in the upper portion of the shaftway and suspending the rope 3. Reference numeral 5 represents an L-shaped guide rail disposed along the shaftway, 6 represents a conductor plate held between a pair of the guide rails 5 and made of aluminum, and 6a represents a joint of the two conductor plates 6. Reference numeral 7 represents a bracket for supporting the guide rail 5 and the conductor plate 6, and 8 represents a wall of a shaftway pit on which the guide rail and the conductor plate are disposed via the bracket 7.
The conductor plates 6 constitute secondary conductors of the linear induction motor respectively disposed on the two sides of the counterweight 2 in a direction in which the elevation member is elevated/lowered, that is, the conductor plates 6 are disposed along the shaftway.
Since the conductor plate 6 has a finite length, a plurality of the conductor plates 6 are joined up at joints 6a so as to have a length which is the same as that of the passage in which the car 1 is elevated/lowered. The joints 6a are disposed at proper joint clearances in order to prevent expansion and bending of the conductor plate 6 made of aluminum due to high temperature in summer.
Reference numeral 9 represents a primary coil of the linear induction motor, thatis, an armature of the same. A pair of the armatures 9 is disposed on each side of the counterweight 2 in such a manner that the two armatures 9 face and hold the conductor plate 6 therebetween so that magnetic flux generated at the time of application of the electric power interlinks the conductor plate 6.
Reference numeral 10 represents a speed sensor for detecting moving speed V of the elevation member, that is, the armature 10, the speed sensor 9 being composed of, for example, a disc, which is rotated while being brought into contact with the conduct or plate 6, and an encoder, which is operated in synchronization with the disc, and the like.
Referring to FIG. 5, reference numeral 11 represents a movable cable connected to the armatures 9 and the speed sensor 10 and suspended in the shaftway, 12 represents a control apparatus to which the moving speed V is supplied via the movable cable 11 and which includes an inverter to be described later, the inverter being arranged to supply AC power to the armatures 9.
FIG. 7 is a structural view which specifically illustrates the control apparatus shown in FIG. 5, in which reference numeral 20 represents a power source for supplying a three-phase alternating current, 21 represents a converter comprising a diode bridge which converts the alternating voltage supplied from the power source 20 into a direct current, and 22 represents a capacitor for smoothing the DC voltage transmitted from the converter 21. Reference numeral 23 represents the inverter comprising a transistor bridge which converts the DC voltage smoothed by the capacitor 22 into an alternating current having a variable voltage and a variable frequency, 24 represents a rectifier for detecting electric current Im to be supplied from the inverter 23 to the armature 9 of the linear induction motor, 25 represents a resistor connected in parallel to the capacitor 22 and consuming regenerated electric power supplied from the armature 9, and 26 represents a transistor switch which is switched on at the time of the regeneration of the electric power so as to cause the resistor 22 to consume the electric power.
Reference numeral 27 represents a control circuit for generating three-phase electric current command Is to be issued to the inverter 23 so as to cause the moving speed V detected and supplied from the speed sensor 10 to coincide with speed command Vs supplied from a speed command generator (omitted from illustration), and 28 represents a PWM circuit for generating PWM signal P with which the transistor disposed in the inverter 23 is turned on/off so as to cause the electric current Im supplied to the armature 9 and the electric current command Is to coincide with each other. FIG. 7 will also be used to describe control apparatuses 12A and 12B according to the present invention.
FIG. 8 is a block diagram which specifically illustrates the control circuit 27 shown in FIG. 7, in which reference numeral 31 represents a subtractor for subtracting the moving speed V from the speed command Vs so as to generate speed deviation .DELTA.V, 32 represents a speed controller for generating propulsion command Fs by compensating the gain and the phase of the speed deviation .DELTA.V, and 33 represents an electric-current command generator for generating the electric current command Is to be supplied to the armature 9 (the PWM circuit 28 in actual fact) in accordance with the propulsion command Fs and the moving speed V.
The electric-current command generator 33 serves as an electric power command generator for controlling the output electric power from the inverter 23, and the electric-current command Is may be replaced by a voltage command.
Then, the operation of the conventional apparatus for controlling an elevator will now be described with reference to FIGS. 5 to 8.
When the speed command V.sub.s has been generated by the speed command generator (omitted from illustration), the speed controller 32 disposed in the control circuit 27 generates the propulsion command Fs in accordance with the speed deviation .DELTA.V, while the electric-current command generator 33 generates the electric-current command Is in accordance with the propulsion command Fs and the moving speed V.
Although the higher the cut-off frequency of the speed control system, which uses the speed controller 32, the better the responsiveness of controlling the elevator, the actual cut-off frequency is set to about several radian/second in order to prevent resonance with the mechanical system of the elevator including the rope 3.
The reason for this lies in that the frequency components larger than the several radian/second must be eliminated because the resonant frequency of the mechanical system of an elevator is usually about tens of radian/second (several Hz).
The PWM circuit 28 generates the PWM signal P in accordance with the electric-current command Is thus obtained, the PWM signal P being used to operate and control the inverter 23 in such a manner that the armature electric current Im coincides with the electric-current command Is.
As a result, eddy currents are generated in the conductor plates 6 due to the interlinked magnetic flux generated by the armatures 9, causing the armatures 9 to move along the conductor plates 6 due to the electromagnetic induction. Hence, the car .1 is elevated/lowered together with the armatures 9 along the rope 3 in accordance with the speed command Vs denoting the desired speed.
However, the presence of the gaps created by the joint clearances 6a between the conductor plates 6a disposed in the shaftway interrupts and discontinues the eddy currents flowing in the conductor plates 6. Therefore, the propulsion can be undesirably changed when the armatures 9 pass through the joint clearances 6a.
Moreover, because the armature 9 has a finite length in the, in which it is moved, peculiar propulsion ripples are generated at the time of the operation of the linear induction motor, causing the car 1 to be jolted and causing the passengers to feel uncomfortable.
The change of the propulsion, which takes place due to the presence of the joint clearances 6a between the conductor plates 6, is two times the slip frequency, while the propulsion ripple peculiar to the linear induction motor is two times the power source frequency, namely, the output frequency (several Hz) from the inverter 23.
Therefore, the frequency of the propulsion change becomes tens of radian/second. However, the propulsion change cannot be compensated because the cut-off frequency of the speed control system is several radian/second as described above.
The conventional apparatus for controlling an elevator, as described above, generates the propulsion command Fs by the speed control system which uses only the speed controller 32 and which has a low cut-off frequency of several radians per second, and controls the inverter 23 in accordance with the electric-current command Is generated in accordance with the propulsion command Fs.
Therefore, the responsiveness of the speed control has been unsatisfactory, and the propulsion change generated due to the presence of the joint clearances 6a and the propulsion ripples peculiar to the linear induction motor cannot be compensated, causing a problem to arise in that satisfactory comfort cannot be provided for the passengers.